1. Field of the Invention
The present invention relates to a process for the formation of dialkyl phosphorochloridothionates by the reaction of chlorine with an alkyl ester of dithiophosphoric acid.
2. Description of the Prior Art
Various processes for the formation of dialkyl phosphorochloridothionates are known. These compounds are valuable chemical intermediates in the manufacture of such products as pesticides, flotation agents, plasticizers, lubricating oil additives, rubber curing compounds, and flame retardants.
In certain prior art patents a two-stage reaction between the alkyl ester of the acid and chlorine has been proposed. For example, in U.S. Pat. No. 3,897,523 to H. E. Sorstokke, substantially all the acid ester is chlorinated in two chlorination zones with the first zone having from 60-93% of the needed chlorine introduced. A recent improvement to such a process (U.S. Pat. No. 4,332,747 to J. W. Watson, Jr. et al., Col. 3, lines 9-19) contemplates dilution of the chlorine gas reagent after substantially all HCl by-product has been evolved to simplify recovery of the HCl. Another two-stage chlorination procedure in which all the acid ester is present in each of two separate chlorination zones is suggested by U.S. Pat. No. 4,075,292 to K. H. Vopel at Col. 3, lines 38-49. In the Volpe reference the second stage chlorination is accomplished solely by the reaction of S.sub.2 Cl.sub.2 formed in the first stage chlorination.
It has also been suggested to chlorinate the acid ester in two separate chlorination zones with only a portion of the acid ester being chlorinated in the first of such zones (U.S. Pat. No. 4,078,023 to B. Lippsmeier et al. ). In the Lippsmeier process, a portion of acid ester present in the first stage is subjected to substantially complete chlorination, due to the use of excess chlorine (Col. 3, lines 9-12 of Lippsmeier et al.). After removal of the sulfur precipitate a distillate product from a second stage chlorination is added thereto along with fresh, unreacted acid ester and this mixture is distilled. In a second stage the distillation residue from the first stage is further chlorinated and the sulfur precipitate separated, and the liquid distilled. The distillate product from this stage is recycled to the first stage. The chlorine supplied to both stages is supplied to the acid ester in diluted form.
In the above described process, the unreacted acid ester added to the first stage, after the initial chlorination is completed, can be hazardous due to the thermal instability of unchlorinated material especially during distillation.
U.S. Pat. No. 4,354,983 to Roszinski discloses a process wherein dialkyl dithiophosphoric acid is chlorinated utilizing 3 moles of Cl.sub.2 per 2 moles of acid and thereafter a further mole of the acid is added prior to recovering the product by distillation. This process presents the same dangers inherent in the Lippsmeier et al. process.
It is an object of the instant invention to improve the yield of the dialkyl phosphorochloridothionate while decreasing the amount of impurities such as disulfur dichloride produced.